It is desirable that drug therapy or, more specifically, the drug fluid being provided from the drug fluid reservoir of an implantable drug pump be provided at a substantially constant rate. Conventional practice achieves the substantially constant rate by use of only a single substance or fluid that acts as a pressure-providing means. The substance typically used to drive such a "gas driven" pump is a fluid that is in phase change between a liquid state and a gas state when, i.e., in equilibrium between phases at around 37 degrees (Celsius), which is the usual temperature of the human body.
Three known examples of fluids that are each used as the single propellant for gas-driven implantable pumps are (1) R-11, a propellant that is used, for example, in the commercially available Infusaid drug pump; (2) R-114, a propellant that is used in commercially available TricuMed drug pumps; and (3) n-butane, a well known and commercially available substance also used in TricuMed implantable drug pumps.
The R-11 propellant supplies approximately 8.5 p.s.i.g. to the drug reservoir of a known drug pump, the Infusaid model, at 37 degrees (Celsius). Two known TricuMed drug pumps use R-114, the 35.1 and 20.1 models. The R-114 propellant supplies about 31 p.s.i.g. at 37 degrees (Celsius) to the reservoirs of those drug pumps. The n-butane propellant, used in the drug reservoirs of TricuMed's version 20.2 and 35.2 drug pumps, provides a reservoir pressure of 36 p.s.i.g. at around 37 degrees (Celsius).
There are, however, many disadvantages that result with the use of either of the above-mentioned single-substance propellants. The disadvantages relate to the preservation of earth's environment, fundamental operability of drug pumps under changing conditions, safety during manufacture, and ease of use during clinical preparations.
Specifically, the known, singly-used propellants R-11 and R-114 are both chloro-fluoro-carbons, also commonly referred to as CFCs. These substances are thought to deplete the protective ozone layer of the atmosphere of the earth. As a result, the use of CFCs is being limited or eliminated throughout the earth. Meanwhile, the diminishing production of such fluids has made them increasingly expensive to obtain and, therefore, to use.
Additionally, use of a propellant such as R-11, which has low pressure as a characteristic property, raises functionality issues. Specifically, an 8.5 p.s.i.g. propellant reservoir pressure, such as is provided using R-11, is so low a pressure that when changes of temperature, altitude and/or drug reservoir volume occur, the propellant reservoir pressure varies substantially, adversely impacting the accuracy of drug delivery.
Use of the propellant R-114, which as stated above is used for a 31 p.s.i.g. pump reservoir pressure, is disadvantageous because of clinical use problems. Specifically, the characteristic pressure of such a propellant is so high that a clinician has extreme difficulty (re)filling the drug reservoir of the implantable drug pump with drug fluid.
Another clinical use problem arises with the use of n-butane as the propellant. This propellant is not a CFC, but it is also a high pressure fluid. The 36 p.s.i.g. reservoir pressure associated with n-butane is even worse than R-114 from the standpoint that the clinician has extreme difficulty (re)filling the pump. Moreover, n-butane is flammable. Thus, since propellant fluid is welded into the drug pump (behind or below the pump's bellows reservoir), use of a flammable substance, such as n-butane, necessarily creates a significant risk of hazard.
In sum, conventional single-substance propellants have forced designers to confront the environmental hazards of CFCs (and also toxicity), and to choose between using either (a) a relatively easier to (re)fill, lower-pressure propellant which offers lower flow rate accuracy, or (b) a relatively much more difficult to (re)fill higher pressure propellant, which offers higher flow rate accuracy.